This invention relates to a telephone exchange accommodating lines having a plurality of channels capable of being handled as a group, wherein the state of use of each line is displayed.
In conventional telephone exchanges, a special-purpose extension telephone is provided with a plurality of LEDs, which correspond to outside-line keys for channel selection, in order to display the state of use of lines having a plurality of channels that are capable of being handled as a group. The state of use of the lines is indicated by the flashing state of the LEDs. An example of the LEDs employed is a two-color LED of, say, the colors red and green. The combination of the emitted colors and the flashing condition makes it possible to indicate the states of use of lines of a plurality of types.
Further, when the LED corresponding to an individual channel, such as channel 1, lights in the color green on a certain extension telephone accommodated in the conventional telephone exchange and the LED corresponding to channel 1 on another extension telephone lights in the color red, control is such that channel 1 is put in at the extension telephone whose green LED is lit.
However, such an arrangement in which an LED is provided for every channel is disadvantageous. For example, in a case where a telephone exchange accommodates a line having a plurality of channels, such as an arrangement in which one line has 23 (or 24) channels, as in the manner of the lines of a primary group of an ISDN, it is required that the number of LEDs be the same as the number of channels despite the fact that the telephone number assigned to the channels is the same. This is not only an impediment to a reduction in the size of the apparatus but is also uneconomical since it leads to a rise in the cost of the apparatus.